A typical ink jet printer has a pen that reciprocates over a printable surface such as a sheet of paper. The pen includes a print head having an array of numerous orifices through which droplets of ink may be expelled onto the surface to generate a desired pattern. The pen includes a body defining an ink chamber containing a supply of ink. Other printer formats may employ a fixed remote supply that supplies ink to a reciprocating pen. In such cases, for the purposes of this disclosure, the remote supply is considered part of the pen.
Many ink jet printing systems employ pens with rigid ink supply reservoirs that must be vented to admit air to compensate for the volume of ink that is consumed during printing. This prevents the development of a partial vacuum that can inhibit further printing. In addition, it also allows equalization of pressure differentials caused changes in temperature and atmospheric pressure. Without a vent, when the ink chamber exceeds ambient pressure, ink may be forced out of the printing orifices, causing associated problems.
Existing ink supply chambers are normally vented by an aperture in which a small cylindrical vent plug is inserted. The plug has a helically grooved exterior surface, with a V-shaped groove that provides an elongated narrow path for gradual equalization of pressure differences. The path is long and narrow enough to prevent significant air exchange in an atmospherically stable environment; a large vent opening would allow pressure differences. The path is long and narrow enough to prevent significant air exchange in an atmospherically stable environment; a large vent opening would allow evaporation and drying of the ink in the reservoir. Such existing plugs are formed in the shape of a headless threaded machine screw, of a plastic material such as nylon. The sharp vertices of the threads are intended to crush slightly to provide a tight fit in the pen housing aperture, preventing air flow other than via the single thread channel.
While effective for venting, existing plug designs occasionally experience difficulties during installation. The plug may become angularly offset initially, and insertion forces can bend the plug or cause damaging crushing of the threads on one side of the plug, blocking the vent channel and causing pen failure when ink consumption generates an unrelieved vacuum in the chamber. In addition, the helically threaded design may generate an uneven buckling response to axial forces, and may respond unevenly to initial insertion where one side of the plug end has a raised thread peak and the other has a groove. Further, the softness of threads and the sharp vertex design needed to allow slight crushing for an interference fit may be damaged by the imbalanced forces of bending, buckling, and offset insertion.
The present invention overcomes the limitations of the prior art by providing an inkjet pen having a body enclosing an ink reservoir and defining a vent aperture. A vent plug defining an elongated serpentine channel occupies the aperture. The plug may be cylindrical, with smaller diameter pilot ends to facilitate insertion. The serpentine channel may include several segments that alternate in direction, and which occupy a major portion of the plug surface away from an elongated spine surface portion that extends the length of the plug without interruption by the channel.